Conventional pulley drives are known which transfer torque via a traction element, such as a drive belt, that extends between a drive pulley and a driven pulley located on the shaft of an accessory drive in order to rotate the shaft. In conventional internal combustion engines, belt-pulley drives are typically used to drive the water pump, the alternator, the power steering pump and the air conditioning compressor, as well as potentially other accessory drive items. One drawback of the conventional arrangement is that the speed that the accessory drive is driven is proportional to the engine speed at all times. Often, the accessory drive devices are not needed or should not operate for certain engine conditions or at a speed proportional to the engine speed.
One alternative to this which is known in the art is to utilize an electric motor that is speed controllable. The electric motor is driven by the vehicle battery and charging system. However, this is expensive and increases the electrical load on the alternator and battery.
It has also been known to use a viscous coupling between the drive pulley and a pump shaft or a water pump. This is disclosed in EP 1 830 045 B1. This viscous coupling between the drive pulley and the pump shaft engages the pump when the coolant temperature rises. One drawback is that this arrangement is entirely dependent on the engine coolant temperature and cannot be engaged manually. The default position is disengaged.
It is also known to use an electromagnetically actuated coupling between the drive pulley and a water pump shaft. This is disclosed in U.S. Pat. No. 5,076,216. The electromagnet engages the water pump shaft to the drive pulley when a control current is applied. This is also an expensive solution and the default position is disengaged. Therefore, in the event of a partial or total electrical system failure, the water pump does not operate, which can result in significant engine damage.
It has also been known to use an electric motor to push a pulley into contact with the drive belt. This is disclosed in U.S. Pat. No. 5,603,289. A shaft is radially moved by a motor to come into or out of contact with an accessory drive belt, or is translated via an Acme screw rotated by the motor. However, this arrangement requires additional space, and either the pulley or belt must be able to be moved. Again, in the event of an electrical system failure, if the pump is not engaged, it will remain disengaged.